Distillation



Nov. 29, 1955 P. LAMBERT 2,725,342

DISTILLATION Filed Aug. 25, 1951 INVENTOR ilnited States Patent DISTILLATION Pierre Lambert, Chappaqua,. N. Y., assignor to The Lummus Company, New York, N. Y., a corporation of Delaware Application August 25, 1951, Serial No. 243,623

3 Claims. (Cl. 196-43).

This invention relates to improvements in petroleum distillation and more specifically, to an improved method and apparatus for topping a crude oil containing a substantial proportion of light ends.

There are two generally used methods for treatment of such crude oil. In the first of these a single distillation column is used, but since temperature limitations in the flash zone preclude operation under pressure and inasmuch as it is necessary in order to condense the light ends with available water at normal temperature levels, the gaseous overhead is compressed. This requires a compressor and an extraamount of equipment to recover the desired gas.

An alternative method involves the use of two distillation columns, in the first of which the gasoline is produced as an overhead product at such pressure as to allow condensation of the gas with the gasoline. Such a tower requires a stripping section to recover all the gasoline and usually a heater is also required for reboiling. The residue is fractionated in the second tower but as it is usually diificult in such operations to produce a clean separation of cuts, the overhead must be returned to the first tower as slop.

The present invention is a variation of the so-called two tower system being primarily characterized in the operation of the first tower at a substantial pressure and with a relatively low temperature such that there is only a partial vaporization of the gas and gasoline. The remaining gas and gasoline together with theside cuts and bottoms is then fractionated in a second column at a substantially lower pressure although at a substantially higher temperature than in the first tower. The overhead heavy ends of the gasoline are then added to the gas and gasoline from the first tower to reconstitute the gasoline yield.

The economies of such an operation are primarily in the elimination of heaters, the use of smaller tower equipment and the simplified control of the end products. There is no need of compressors or refrigerant cooling and the unit is flexible in operation with changes in feed stock.

Further objects and advantages of my invention will appear from the attached drawing illustrative thereof, such drawing being a schematic flow diagram of a part of a topping unit for recovering gasoline from crude oil.

As more particularly shown in the drawing, an unreduced crude such as Kuwait containing approximately 2% gas, about 14.5% gasoline, with side streams account for 43.5% and bottoms amount to about 40%, is charged at through pump 11 and heat exchanger 12 to the topping unit 14. Usually the crude is passed through a desalting operation at 16 and it may also pass through another heat exchanger 18.

In accordance with my invention, topping unit 14 which is in the nature of a flash absorber, has a lower flash zone and fractionating decks above the point of feed and is operated at a superatmospheric pressure in the order of 30-50 p. s. i. g. The bottom temperature of this particular tower was maintained in the range of 300 F.400 F. to flash ofl an overhead amounting to about 7 /2% of the crude. This includes approximately 1.8% of the total gas and about 5.7% of the gasoline or approximately half of the total gas and gasoline in the crude.

The overhead removed through line 20 is condensible with water in the heat exchanger 22 and the condensate is collected in drum 24. Some of the condensate may be pumped through pump 25 and line 26 as reflux to the top of tower 14 and the remainder removed at 28, such material being of the nature of straight run gasoline and gases up to C4. Normally such product is ultimately debutanized and may be depropanized with the butane returned back to the gasoline in a separate system to provide the specification vapor pressure gasoline.

The bottoms from tower 14 are removed through. line 30 and pass through heat exchanger 32 and fired heater 34 where the temperature is brought up to approximately 700 F. to 750 P. which is substantially below cracking. The heated feed is then introduced into the second tower 36 which is operated at a substantially lower pressure in the order of 10-15 p. s. i. g.

In the second tower, the heavy ends of the gasoline driven overhead through line 40 are condensible in heat exchanger 42 with water. It will be appreciated that the heavy ends of the gasoline will condense ata higher temperature than the entire gasoline fraction. The condensate is collected at 44 and some of it removed through line 46, is returned to column 36 as reflux. The side streams removed from this tower account for approximately 43.5% of the crude and the bottoms removed through the line 47 account for about 40% of the crude charged at 10. Stripping steam is introduced through the line 38 to this tower.

The condensate in drum 44 includes the heavy ends of the gasoline which is returned through line 48 to the first tower to which it may be introduced, in part through line 48a to the overhead vapors in line 20, and in part through line 4812 through the reflux line to the top deck. By returning such heavy ends to the already recovered light ends, the line 28 includes all of the gasoline and gas in the crude.

It will be apparent that under the circumstances, re covery of the light ends of the gas and gasoline is accomplished in the first tower at temperatures lower than customary and with pressures which would otherwise cause cracking in a one tower system. In the second step the separation is carried out at the necessary high temperature to recover heavier ends of the gas and gasoline but at pressures which can be substantially lower and in a tower which is substantially smaller than customary, due to the absence of the first 7%. of the crude. As a result both operations can be carried out with water cooled heat exchangers without the need of external compressors or other devices.

While I have shown and described a preferred form of embodiment of my invention, I am aware that modifications may be made thereto and I desire a broad interpretation of the invention within the scope and spirit of the description herein and of the claims appended hereinafter.

I claim:

1. The method of topping crude oil containing substantial portions of gas and gasoline to produce maximum gasoline boiling range product which comprises, preheating the crude oil and passing the heated oil to first stage distillation carried out at a pressure above atmospheric pressure at'which about one half of the total gas and gasoline content of the crude oil will be vaporized in flashing and rectification operations carried out in the first stage distillation, withdrawing as overhead gas and gasoline vapors from the first stage distillation, and condensing said vapors to produce a condensate, refluxing first stage distillation with a portion of said condensate and withdrawing the remainder of said condensate as a part of the total gasoline boiling range product, withrawing a bottoms product from first stage distillation, heating said bottoms product to a temperature substartially above the temperature .at which the'first stage di tillation was carried out, passing said hot bottoms to a second stage distillation carried-out at a-pressure below the pressure of the first stage distillation, withdrawing gasoline as overhead from second stage distillation and condensing said gasoline to produce a liquid gasoline condensate, refluxing said second stage distillation with a portion of gasoline condensate, combining a second portion of gasoline condensate with :the reflux stream of condensed overhead from the first stage distillation used in refluxing said first stage distillation, combining a third portion of gasoline condensate with the overhead Cr d first stage distillation to effect simultaneous preliminary condensation of said overhead and to absorb the normally gaseous portion of the first stage distillation overhead and withdrawing the .condensed gas and gasoline as total gasoline boiling range product.

2. The method of separating the components of a hydrocarbon mixture which includes gas and gasoline to recover maximum gasoline boiling range product which comprises, heating the hydrocarbon mixture and passing it to a primary fractionating column to obtain an overhead and a residue, maintaining :said column at a temperature and pressure which permits substantialiy com plete removal of the gas components of said mixture in the overhead and partial removal of the gasoline components of the mixture in the overhead, withdrawing said overhead from the primary fractionating column, indirectly condensing said overhead, returning a portion of the condensed overhead as reflux in the primary column and withdrawing the remaining portion of condensed overhead as a part of the total gasoline product, withdrawing residue as a bottoms product from said primary fractionating column, heating said residue to a temperature higher than the bottoms temperature of said primary fractionating column and passing the heated residue to a secondary distillation column, operating said column at a lower pressure than the primary distillation column, withdrawing the remaining heavy ends of the gasoline as an overhead from said secondary distillation column, indirectly condensing said overhead and refluxing said secondary column with a first portion of said condensed overhead, returning a second portion of the condensed overhead from the second column as combined reflux in the primary column, introducing a third portion of the condensed overhead from the secondary column into the overhead stream of the primary column before indirect condensation of said stream to provide simultaneous direct condensation and absorption of the gas components of said overhead to provide a total gasoline product.

3. The method of claim 2 wherein the first separation is at a pressure in the range of 30 to 50 p. s. i. g. and between 300 F. and 400 F. bottom temperature and the second separation is at apressure in the range of 10 to 15 p. s. i. g. and at a temperature between 700 F. and 750 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,877,811 Coleman Sept. 20, 1932 2,039,835 Petersen May 5, 1936 2,105,935 Swanson Jan. '18, 1938 2,149,058 Kraft Feb. 28, 1939 

1. THE METHOD OF TOPPING CRUDE OIL CONTAINING SUBSTANTIAL PORTIONS OF GAS AND GASOLINE TO PRODUCE MAXIMUM GASOLINE BOILING RANGE PRODUCT WHICH COMPRISES, PREHEATING THE CRUDE OIL AND PASSING THE HEATED OIL TO FIRST STAGE DISTILLATION CARRIED OUT AT A PRESSURE ABOVE ATMOSPHERIC PRESSURE AT WHICH ABOUT ONE HALF OF THE TOTAL GAS AND GASOLINE CONTACT OF THE CRUDE OIL WILL BE VAPORIZED IN FLASHING AND RECTIFICATION OPERATIONS CARRIED OUT IN THE FIRST STAGE DISTILLATION, WITHDRAWING AS OVERHEAD GAS AND GASOLINE VAPORS FROM THE FIRST STAGE DISTILLATION, AND CONDENSING SAID VAPORS TO PRODUCE A CONDENSATE, REFLUXING FIRST STAGE DISTILLATION WITH A PORTION OF SAID CONDENSATE AND WITHDRAWING THE REMAINDER OF SAID CONDENSATE AS A PART OF THE TOTAL GASOLINE BOILING RANGE PRODUCT, WITHDRAWING A BOTTOMS PRODUCT FROM FIRST STAGE DISTILLATION, HEATING SAID BOTTOMS PRODUCT TO A TEMPERATURE SUBSTANTIALLY ABOVE THE TEMPERATURE AT WHICH THE FIRST STAGE DISTILLATION WAS CARRIED OUT, PASSING SAID HOT BOTTOMS TO A SECOND STAGE DISTILLATION CARRIED OUT AT A PRESSURE BELOW THE PRESSUREE OF THE FIRST STAGE DISTILLATION, WITHDRAWING GASOLINE AS OVERHEAD FROM SECOND STAGE DISTILLATION AND CONDENSING SAID GASOLINE TO PRODUCE A LIQUID GASOLINE CONDENSATE, REFLUXING SAID SECOND STAGE DISTILLATION WITH A PORTION OF GASOLINE CONDENSATE, COMBINING A SECOND PORTION OF GASOLINE CONDENSATE WITH THE REFLUX STREAM OF CONDENSED OVERHEAD FROM THE FIRST STAGE DISTILLATION USED IN REFLUXING SAID FIRST STAGE DISTILLATION, COMBINING A THIRD PORTION OF GASOLINE CONDENSATE WITH THE OVERHEAD OF SAID FIRST STAGE DISTILLATION TO EFFECT SIMULTANEOUS PRELIMINARY CONDENSATION OF SAID OVERHEAD AND TO ABSORB THE NORMALLY GASEOUS PORTION OF THE FIRST STAGE DISTILLATION OVERHEAD AND WITHDRAWING THE CONDENSED GAS AND GASOLINE AS TOTAL GASOLINE BOILING RANGE PRODUCT. 